Children or adults with severe congenital T cell defects have abnormalities of in T cell activation in vitro responsive to added IL-2. Since IL-2 can substantially boost T cell function in vitro, a clinical trial was recently conducted to determine if IL-2 could improve T cell functions and the clinical state of infants with SCID phenotype. For 5 of 6 patients, remarkable clinical and immunologic benefit was observed. Since many patients with common variable immunodeficiency have reduced IL-2 production and substantial T cell defects which correctable in vitro by the addition of IL-2, we have established a new collaborative clinical trial with Cetus Corporation to provide us with a new biologic, a recombinant IL-2 conjugated to polyethylene glycol; this coupling procedure results in an IL-2 compound with greatly increased half-life and no loss of biologic activity. In the Cetus protocol we will treat 3 -> 4 pediatric patients with severe T cell defects and 10 - 12 adult and pediatric patients with common variable immunodeficiency (CVI) with T cell defects all of whom will be pre-tested for evidence of an in vitro response to PEG-IL-2. The Cetus protocol includes analyses of clinical state, basic laboratory tests, immunoglobulin and complement levels, enumeration of T and B cell numbers, lymphocyte proliferative responses to mitogens, antigens, allogeneic cells, evaluation of IL-2 production, soluble IL-2 receptors, and skin test reactivity over a 12 week period. What the Cetus protocol does not address is whether B cell activation and differentiation to immunoglobulin production can be detected after PEG-IL-2 treatment and if cytokines other than IL-2 can be produced by T lymphocytes of treated patients. We have recently demonstrated, in two independent systems, that the concept of an irrevocable intrinsic B cell defect to explain the hypogammaglobulinemia in patients with common variable CVI is probably not valid. Our data suggest that, given the appropriate stimulus form T cells, B cells from these patients can secrete immunoglobulin in vitro at normal or near normal levels. Thus, the Ig deficiency may result from improper "communication" between CVI T cells and autologous B cells. The purpose of this application is to determine whether improved T cell immunity is linked in vitro and/or in vivo to B cell function in these two patient populations. Thus, the purpose of this application is two fold: 1) we will determine if improved T cell proliferation after IL-2 treatment is linked in vitro to enhanced B cell function and 2) if lymphokine secretion by T cells and monocytes in vitro is altered by this treatment.